This disclosure generally relates to systems and methods for imaging objects at long range through the atmosphere, and in particular air-air surveillance and air to space surveillance. In particular, this disclosure relates to systems and methods for compensating for the effects of atmospheric turbulence in images of objects at long range, including stars, orbiting objects, missiles, and airplanes.
When light from a star or another astronomical object enters the Earth's atmosphere, atmospheric turbulence (introduced, for example, by different temperature layers and different wind speeds interacting) can distort and move the image in various ways. Images produced by any telescope larger than 10 cm in diameter are blurred by these distortions. The blur changes rapidly, so that in long-exposure images the higher spatial frequencies are wiped out. One way of dealing with this problem is to correct the wavefront using real-time adaptive optics (AO). A complementary approach is to use speckle imaging techniques, in which an image is reconstructed from many short exposures.
An AO system may comprise a wavefront sensor (WFS) which takes some of the astronomical light, a deformable mirror that lies in the optical path, and a computer that receives input from the detector and controls the surface of the deformable mirror. Alternatively the deformable mirror could be substituted by other types of devices that provide a capability to adjust the phase of an optical beam as a function of spatial position across the beam and as a function of time, another example being a liquid crystal spatial light modulator. The AO system relies on the wavefront sensor to measure turbulence-induced perturbations on the wavefront. These measurements can be used to improve the performance of the imaging system by reducing the effect of wavefront distortions: it aims at correcting the deformations of an incoming wavefront by rapidly deforming a mirror in order to compensate for the distortion.
The simplest form of adaptive optics is tip-tilt correction, which corresponds to correction of the tilts of the wavefront in two dimensions (equivalent to correction of the position offsets for the image). Rapid optical tilts in both X and Y directions are termed jitter. Jitter can arise from rapidly varying three-dimensional (3-D) refraction in aerodynamic flow fields. Jitter may be compensated in an AO system using a flat steering mirror mounted on a dynamic two-axis mount that allows small, rapid, computer-controlled changes in the mirror X and Y angles. By reflecting light from a computer-driven flat steering mirror, an image or laser beam can be stabilized. Image blurring due to motion and far-field laser beam jitter are reduced if not eliminated.
Adaptive optics is inherently an isoplanatic technology, i.e., it assumes that the turbulence effect is constant over an angle termed the “isoplanatic angle”. In fact, the wavefront distortion due to turbulence is spatially varying due to anisoplanatic conditions, while the isoplanatic angle varies with atmospheric conditions. Adaptive optical correction of an extended region consisting of multiple isoplanatic patches (i.e., temporally coherent paths) requires significant added complexity in the AO system.
It would be advantageous to provide astronomical instrumentation that can perform high-quality anisoplanatic imaging over an extended field-of-view using a large aperture without the added complexity of AO systems.